Method for forming a nitridized interface on a semiconductor substrate

ABSTRACT

A surface treatment method for forming a fluorine-doped nitridized interface on a semiconductor substrate. The fluorine-doped nitridized interface may be formed using an ammonia plasma CVD process having a treatment gas doped with a fluorine component, such as carbon hexafluorine. The method may be employed as part of a LOCOS-based processing scheme in the manufacture of MOS semiconductor devices, such as DRAM devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/299,969, filed Apr. 26, 1999, now abandoned. The above notedapplication is hereby incorporated by reference herein in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for forming a nitridized interface ona substrate, and more particularly to improving uniformity of an ammoniaplasma surface treatment on a semiconductor substrate. Specifically,this invention relates to forming a nitridized polysilicon interfaceusing ammonia plasma doped with fluorine.

2. Description of Related Art

Silicon nitride films are commonly employed in the fabrication ofcircuits for use in modern semiconductor devices, such as thefabrication of metal-oxide semiconductor (“MOS”) devices for highdensity integrated circuits and submicron designs. For example, siliconnitride films are employed in the manufacture of MOS devices using LocalOxidation of Silicon processes (“LOCOS”), as well as advanced LOCOSmethods such as polysilicon buffered LOCOS processes (“PBLOCOS”).Further information on LOCOS-based processing technology may be found inWolf.

In a LOCOS manufacturing method, a pad of native oxide is typicallyformed on a semiconductor substrate for purposes of cushioningtransition of stress between a silicon substrate and a silicon nitridefilm, which is deposited to serve as an oxidation mask. Such pad oxidesmay be thermally-grown or deposited using chemical vapor deposition(“CVD”). In a PBLOCOS method, a thin pad layer of thermally depositedsilicon dioxide (SiO₂) in combination with a polysilicon buffer layer isformed. PBLOCOS methods are typically utilized to enhance suppression oflateral oxidation and to provide a stress buffer layer between the oxidelayer and a subsequently deposited silicon nitride layer.

In a typical LOCOS-based process, silicon nitride and polysilicon bufferlayers (when present) are removed selectively to expose those areaswhere field oxide growth is desired, leaving the active areas of thedevice covered. Field oxide is then grown “locally” in the etched areasbetween the active areas covered by silicon nitride film to isolate themfrom each other. It is desirable to minimize the space required forthese isolation zones, as they consume valuable semiconductor space.

It is at the time of field oxide growth that encroachment of oxide intothe silicon substrate and interface area between the surrounding siliconnitride and polysilicon buffer layer (if present) typically occurs. Thisoxide encroachment is commonly referred to as a “first bird's beak” whenit extends into the substrate, and as a “second bird's beak” when itoccurs in the interface of the silicon nitride and polysilicon. Bird'sbeak formation can change the active area size and potentially causegate poly bridging. First bird's beak areas consume additional space asthey extend beyond the edges of the isolation zones, and work againstthe achievement of isolation requirements for submicron devices. Secondbird's beak areas tend to interfere with removal of polysilicon in aLOCOS-based process, leaving undesirable “stringers” of poly which maycause shorts and/or leakage. Following field oxide growth, the remainderof the silicon nitride layer and any buffer layer present underneath istypically removed in order that the active areas of the semiconductordevice may be formed.

Substrate surfaces are typically nitridized using a CVD process such asammonia-plasma CVD. During surface nitridization in a plasma enhancedCVD reactor, flow of reactant gasses, as well as reacted byproducts,typically results in nonuniform film thickness. Nonuniformity oftreatment may be gauged or measured in terms of both film within waferthickness nonuniformity, and in wafer to wafer nonuniformity. Filmwithin wafer thickness nonuniformity may be expresses in terms ofpercentage standard deviation. Wafer to wafer nonuniformity typicallyaverages from about 6 Å to about 10 Å.

Increased film nonuniformity typically results in increased secondbird's beak formation. Such variations in film treatment tend todecrease wafer yield by increasing the second bird's beak size, leadingto poly stringer formation and reduced process capability as measured ona test wafer run with the product. Process capability is defined as theprocess spec width divided by (6ε), and is typically expressed as C_(p)or C_(pk).

As an example of problems encountered with conventional LOCOS-basedprocesses, FIG. 1 illustrates field oxide 18 formed on a semiconductorsubstrate 10 in isolation area 11, using a conventional LOCOS-basedprocess known in the art. As shown in FIG. 1, isolation area 11 isdefined between active device areas 13. Active device areas 13 arecovered by silicon dioxide pad layer 12, polysilicon buffer layer 14 andsilicon nitride layer 16. As may be seen in FIG. 1, field oxide 18extends into active areas 13 due to encroachment of oxide 18 into thesubstrate 10, forming “first bird's beak” areas 17, and between siliconnitride layer 16 and polysilicon buffer layer 14, forming “second bird'sbeak” areas 19. Upon subsequent removal of layers 16, 14 and 12,residual areas of polysilicon buffer layer 14 may remain between the“bird's beak” areas 17 and 19, which tends to act as a mask againstremoval of polysilicon. These residual polysilicon areas are referred toas “poly stringers” and are undesirable due to their tendency to causeshorts and leakage.

SUMMARY OF THE INVENTION

Using the disclosed method, a fluorine doped nitride surface treatmentmay be employed to form a fluorine-doped nitridized substrate interface.Benefits of the disclosed method include, but are not limited to,reduction in nonuniformity of nitridized substrate surfaces and wafer towafer nonuniformity, as well as the provision of a more stable process.By increasing uniformity of substrate nitridization, wafer yield may beincreased over conventional undoped ammonia plasma treatment processesby, for example, substantially eliminating across-wafer nonuniformityand the presence of a “second bird's beak” in the field oxide edge areasformed in a LOCOS-based process. Advantageously, in one embodiment ofthe disclosed method, process capability (C_(p), C_(pk)) is increasedbecause of this reduction in thickness variation, and/or formation ofpoly stringers is suppressed or substantially prevented.

While not wishing to be bound by theory, it is believed that lateraloxidation of the poly/nitride interface during field oxidation isprevented by tying up available Si atoms at the surface of the poly.This is believed to result due to the breaking of stressed Si—O—Si bondsat the polysilicon surface by HF formation and surface reaction, directF reaction, and/or bombardment. When this occurs, Si—O—Si bonds arebelieved to be replaced by Si—F, non-bridging Si—O, and/or Si—N bonds.Formation of Si—N bonds is believed to be the predominate reaction, andit is believed that these bonds create a more uniform interface betweena polysilicon layer and a subsequent silicon nitride layer. Furthermore,fluorine is also believed to break Si—H bonds and Si—OH bonds which mayform in NH₃ plasma, thus tending to form stronger Si—F bonds. It isbelieved that one or more of the previously described mechanisms retardlateral diffusion and reaction of oxygen during field oxide growth, thusyielding better uniformity.

In one embodiment of the disclosed method, a source of fluorine,typically carbon hexafluorine, C₂F₆ (Halocarbon-116), is introduced intoan ammonia plasma to nitridize a substrate surface (such as oxide orpolysilicon) in a LOCOS-based process, for example, a LOCOS or PBLOCOSisolation scheme. LOCOS-based processes are known in the art and aredescribed, for example, in Wolf, Stanley Silicon Processing for the VLSIEra, Volume 2—Process Integration, Lattice Press, Sunset Beach, Calif.,pp. 12-41, 1990, which is incorporated herein by reference.

In one embodiment, C₂F₆ gas (typically employed as an etchant) isintroduced into an ammonia plasma during nitride surface treatment ofpolysilicon. In this embodiment, addition of fluorine-based dopant maysurprisingly be used to reduce film within wafer thickness nonuniformityto less than or equal to about 2 Å (or about 1σ), as measured on a testwafer, resulting in reduction in wafer to wafer nonuniformity andproviding a more stable process.

Advantages of the disclosed method may be realized in any semiconductorfabrication surface treatment process employing silicon nitride. Forexample, the disclosed method may be employed in the manufacture of MOSsemiconductor devices on a silicon substrate including, but not limitedto, manufacture of DRAM devices.

In one respect, disclosed is a method of nitridizing the surface of asemiconductor substrate, including forming a fluorine-doped nitridizedsurface on an upper surface of the substrate, such that an interface isdefined between the fluorine-doped nitridized surface and the substratesurface. The forming may include exposing the substrate surface to atreatment gas including a fluorine component and a nitrogen component;such that the exposure results in the formation of a fluorine-dopednitridized surface having an interface with the substrate surface. Thesubstrate surface may include silicon dioxide or polysilicon. In oneembodiment, the nitrogen component may be at least one of ammonia,nitrogen, or a mixture thereof; the fluorine component may be at leastone of C₂H₆, C₃F₈, CF₄, or a mixture thereof; and the exposing may occurin a chemical vapor deposition process. In another embodiment, thenitrogen component may be ammonia; the fluorine component may be carbonhexafluorine; and the exposing may occur in a low pressure plasmaenhanced chemical vapor deposition process. The fluorine-dopednitridized surface may include a film may have a thickness of at leastabout 1 Å to about is 40 Å, alternatively from about 10 Å to about 30 Å.The fluorine-doped nitridized surface may be formed using a treatmentgas including a volume ratio of carbonhexafluorine to ammonia of fromabout 1:1 to about 1:20.

The method may further include exposing the fluorine-doped nitridizedsurface to an undoped treatment gas including a nitrogen component; suchthat the exposure results in the formation of an undoped nitridizedsurface on the fluorine-doped nitridized surface. In such a case, thenitrogen component of the fluorine doped treatment gas may be ammonia;the fluorine component of the fluorine-doped treatment gas may be carbonhexafluorine; the nitrogen component of the undoped treatment gas may beammonia; and the exposing to the fluorine-doped treatment gas and theexposing to the undoped treatment gas each may occur in a respective lowpressure plasma enhanced chemical vapor deposition process. Thefluorine-doped nitridized surface may include a film may have athickness of from about 1 Å to about 40 Å; and the undoped nitridizedsurface may include a film may have a thickness of from about 700 Å toabout 3000 Å. The substrate may include a semiconductor wafer, and anupper surface of the undoped nitridized surface may have a within waferthickness variation of less than about 2 Å.

In another respect, disclosed are MOS semiconductor devices (such asDRAM memory devices), and methods for forming localized field oxideduring fabrication of such devices on a substrate having a silicondioxide pad layer present on an upper surface of the substrate and apolysilicon buffer layer disposed on an upper surface of the silicondioxide pad layer, including forming a fluorine-doped nitridized surfaceon an upper surface of the polysilicon buffer layer; such that aninterface is defined between the fluorine-doped nitridized surface andthe upper surface of the polysilicon buffer layer; forming an undopednitridized surface on the fluorine-doped nitridized surface, the undopednitridized surface and the fluorine-doped nitridized surface togetherforming a silicon nitride layer; defining at least one active regionpattern on the silicon nitride layer; removing the silicon nitride andthe polysilicon buffer layers in an area outside the active regionpattern; and forming a field oxide region in the area where the siliconnitride layer and polysilicon buffer layers have been removed. Theformation of a fluorine-doped nitridized surface may include exposingthe upper surface of the polysilicon buffer layer to a treatment gasincluding a mixture of carbon hexafluorine and ammonia in a low pressureplasma enhanced chemical vapor deposition process. The formation of anundoped nitridized surface may include exposing the fluorine-dopednitridized surface to a treatment gas including ammonia andsubstantially no fluorine component in a low pressure plasma enhancedchemical vapor deposition process. The fluorine-doped nitridized surfacemay include a film may have a thickness of at least about 10 Å to about30 Å, alternatively from about 20 Å to about 25 Å. The undopednitridized surface may have a thickness of from about 700 Å to about3000 Å, alternatively from about 1200 Å to about 1800 Å. Thefluorine-doped nitridized surface may be formed using a treatment gasincluding a volume ratio of carbonhexafluorine to ammonia of from about1:3 to about 1:20. The substrate may include a semiconductor wafer, andan upper surface of the undoped silicon nitride layer may have a withinwafer thickness variation of less than about 2 Å. In one embodiment,encroachment of oxide into the interface defined between thefluorine-doped nitridized surface and the upper surface of thepolysilicon buffer layer may be inhibited or substantially preventedduring formation of the field oxide region.

In another respect, disclosed are MOS semiconductor devices (such asDRAM memory devices), and methods for forming localized field oxideduring fabrication of such devices on a silicon substrate, includingforming a pad layer of silicon dioxide on the silicon substrate; forminga buffer layer of polysilicon on the silicon dioxide pad layer, the padlayer of silicon dioxide being disposed between the buffer layer ofpolysilicon and the silicon substrate; forming a fluorine-dopednitridized surface on an upper surface of the polysilicon buffer layer;wherein an interface may be defined between the fluorine-dopednitridized surface and the upper surface of the polysilicon bufferlayer; forming an undoped nitridized surface on the fluorine-dopednitridized surface, the undoped nitridized surface and thefluorine-doped nitridized surface together forming a silicon nitridelayer; defining at least one active region pattern on the siliconnitride layer, removing the silicon nitride layer and the polysiliconbuffer layer in the area outside the active region pattern; and forminga field oxide region on the silicon substrate in the area where thesilicon nitride layer and polysilicon buffer layers have been removed.Formation of a fluorine-doped nitridized surface may include exposingthe upper surface of the polysilicon buffer layer to a treatment gasincluding a mixture of carbon hexafluorine and ammonia in a low pressureplasma enhanced chemical vapor deposition process; and formation of anundoped nitridized surface may include exposing the fluorine-dopednitridized surface to a treatment gas including ammonia andsubstantially no fluorine component in a low pressure plasma enhancedchemical vapor deposition process. In one embodiment, a fluorine-dopednitridized surface-may include a film may have a thickness of from about20 Å to about 25 Å, and/or an undoped nitridized surface may have athickness of from about 1200 Å to about 1800 Å. The fluorine-dopednitridized surface may be formed using a treatment gas including avolume ratio of carbonhexafluorine to ammonia of from about 1:3 to about1:15. Using one embodiment of this method, an upper surface of thesilicon nitride layer surface may have a within wafer thicknessvariation of less than about 2 Å. In another embodiment, encroachment ofoxide into the interface defined between the fluorine-doped nitridizedsurface and the upper surface of the polysilicon buffer layer may beinhibited or substantially prevented during the formation of the fieldoxide region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified partial cross-sectional view of a semiconductorsubstrate showing field oxide regions formed using a conventionalLOCOS-based process, and exhibiting first and second “bird's beak” oxideencroachment beneath adjacent silicon nitride mask layers.

FIG. 2 is a simplified partial cross-sectional view of a semiconductorsubstrate with a pad oxide layer formed thereupon according to oneembodiment of the disclosed method.

FIG. 3 is a simplified partial cross-sectional view of a semiconductorsubstrate with a pad oxide layer and polysilicon buffer layer formedthereupon according to one embodiment of the disclosed method.

FIG. 4 is a simplified partial cross-sectional view of a semiconductorsubstrate with a pad oxide layer, polysilicon buffer layer,fluorine-doped nitridized polysilicon interface and fluorine-dopedsilicon nitride film formed thereupon according to one embodiment of thedisclosed method.

FIG. 5 is a simplified partial cross-sectional view of a semiconductorsubstrate with an oxide pad layer, polysilicon buffer layer,fluorine-doped nitridized polysilicon interface, fluorine-doped siliconnitride film and undoped silicon nitride film according to oneembodiment of the disclosed method.

FIG. 6 is a simplified partial cross-sectional view of a semiconductorsubstrate of FIG. 5 with doped and undoped silicon nitride films andpolysilicon buffer layers removed in preparation for forming field oxideregions according to one embodiment of the disclosed method.

FIG. 7 is a simplified partial cross-sectional view of a semiconductorsubstrate of FIG. 6 showing a field oxide region grown in the area wheredoped and undoped silicon nitride films and polysilicon buffer layershave been removed according to one embodiment of the disclosed method.

FIG. 8 is a simplified partial cross-sectional view of the semiconductorsubstrate of FIG. 7 showing doped and undoped silicon nitride filmsremoved over the active areas of the semiconductor substrate accordingto one embodiment of the disclosed method.

FIG. 9 is a simplified partial cross-sectional view of the semiconductorsubstrate of FIG. 8 showing polysilicon buffer and oxide pad layersremoved over the active areas of the semiconductor substrate accordingto one embodiment of the disclosed method.

FIG. 10 is a simplified schematic of a plasma-CVD reactor setup used incollecting data for Examples 1-4.

FIG. 11 is a contour map of silicon nitride film thickness for the waferof slot 1 of Example 3.

FIG. 12 is a contour map of silicon nitride film thickness for the waferof slot 3 of Example 3.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A fluorine-doped nitridized interface may be formed on a substratesurface using any method suitable for treating a substrate surfaceincluding, but not limited to, CVD. In one embodiment, fluorine-dopedammonia plasma may be used to treat a polysilicon surface using a CVDprocess. In this regard, any CVD-based method suitable for nitridesurface treating may be employed including, but not limited to, plasmaenhanced CVD (“PECVD”), non-plasma enhanced CVD, radiant energy CVD, lowpressure CVD (“LPCVD”), atmospheric CVD, etc. It will be understood withbenefit of the disclosed method that the term “CVD” also includes anypossible combination of CVD types, for example, low pressure plasmaenhanced chemical vapor deposition. Suitable equipment includes any CVDequipment suitable for nitride surface treatment of substrate surfaces.Examples include, but are not limited to, commercially available fromsuppliers such as is Novellus, Applied Materials, Genus, etc. Particularexamples include, but are not limited to, equipment available fromNovellus (e.g., “CONCEPT II”), Applied Materials (e.g., “P5000”,“CENTURA”), and Matson.

A fluorine-doped nitride surface treatment may be performed using anyCVD treatment gas mixture suitable for supplying nitrogen and fluorinecomponents to form a nitridized interface with a substrate surface. Forexample, suitable nitrogen gas components include, but are not limitedto, at least one of ammonia, nitrogen, or a mixture thereof. Suitablefluorine gas components include, but are not limited to, at least one ofcarbon hexafluorine (C₂H₆), C₃F₈, carbon tetrafluoride (CF₄), or amixture thereof.

In one embodiment of the disclosed method, uniformity of a nitridesurface treatment may be improved by using a fluorine-doped ammoniaplasma to form a fluorine-doped nitridized interface with a substratesurface. Fluorine-doped ammonia plasma of the disclosed method may beadvantageously employed in any semiconductor manufacturing method whichutilizes ammonia plasma surface treatments. For example, fluorine-dopedammonia plasma may be employed in the manufacture of MOS devices such aslogic devices, microprocessors or memory devices including DRAMs, SRAMs,and ROMs. In one specific example, a fluorine-doped silicon nitridelayer may be formed by a low pressure plasma enhanced chemical vapordeposition process using a treatment gas comprising a mixture of ammoniaand carbon hexafluorine (C₂H₆, or “Halocarbon-116”).

Fluorine and nitrogen gas components may be present in a CVD treatmentgas in any amount suitable for forming a fluorine-doped nitridizedinterface. In one exemplary embodiment of the disclosed method, a volumeratio of carbon hexafluorine to ammonia of from about 1:1 to about 1:20,alternatively from about 1:3 to about 1:20, alternatively from about 1:3to about 1:15, alternatively from about 1:5 to about 1:15, alternativelyfrom about 1:7 to about 1:15, and further alternatively about 1:10, maybe employed. However ratios greater than about 1:20, and less than about1:5 are also possible. A treatment gas may optionally include othercomponents if so desired. In this regard, any other CVD gas componentsknown in the art to be suitable for use in the formation of siliconnitride films may be employed including, but not limited to, nitrogen.In those cases where other gas components are present, it will beunderstood that carbon hexafluorine and ammonia may be present in theratios described above, with ammonia and other gas constituents, such asnitrogen, making up the remainder of the treatment gas volume.

An undoped second nitride surface treatment may be performed followingcreation of a fluorine-doped nitridized interface with a fluorine dopedfirst nitride surface treatment. With benefit of this disclosure, suchan undoped nitride surface treatment may be performed using any suitablemethods known in the art, including CVD methods described elsewhereherein. As used herein in relation to treatment gas composition or filmcomposition, “undoped” means containing substantially no fluorine atomcontent, and “fluorine-doped” means containing sufficient fluorine atomcontent to result in or to form a nitridized interface with a substratesurface that inhibits or substantially prevents formation of a secondbird's beak as described elsewhere herein.

In one embodiment, a fluorine-doped nitridized interface may be formedon a substrate during a first nitride surface treatment by using a CVDtreatment gas containing both fluorine and nitrogen components. In thisregard, exposure to a fluorine component need only be of sufficientduration to create a fluorine-doped nitridized interface on a substratesurface that is capable of retarding or preventing lateral diffusion andreaction of oxygen at the nitridized interface during, for example,field oxide growth in a LOCOS-based process. An undoped second nitridesurface treatment may follow the first treatment, and may be performedusing an undoped CVD treatment gas having a nitrogen component, such asammonia. In one embodiment, such a second treatment is performed in aseparate step, for example, in a separate reactor or in the same reactorafter reconfiguration for processing with the undoped treatment gasmixture. Alternatively, when a substrate surface has been sufficientlyexposed to a fluorine doped CVD treatment gas so as to form afluorine-doped nitridized interface of desired characteristics, thefluorine component of the treatment gas may be shut off, with processingcontinuing with a treatment gas containing a nitrogen component. Thelatter case may be possible, for example, using a reactor system inwhich stoichiometrically correct amounts of treatment gas components maybe supplied for both doped and undoped treatment steps. Such a reactorsystem includes, for example, a Novellus “CONCEPT II”.

When using the disclosed fluorine-doped nitride surface treatment toform a nitridized interface on a substrate surface, an associatedfluorine-doped nitride film may be formed. It will be understood withbenefit of this disclosure that as long as a fluorine-doped nitridizedinterface is formed on the surface, the thickness of an accompanyingfluorine-doped nitride film may vary as desired. In one exemplaryembodiment for the fabrication of MOS devices using LOCOS-basedprocessing, overall thickness of a fluorine-doped nitride film layerformed on a polysilicon surface may be from about 1 Å to about 40 Å,alternatively from about 10 Å to about 30 Å, alternatively from about 20Å to about 25 Å, and further alternatively about 23 Å. However, it willbe understood that film thicknesses greater that about 40 Å or less thanabout 1 Å are also possible.

FIGS. 2-9 illustrate formation of field oxide on a substrate using oneexemplary embodiment of the disclosed method in which a PBLOCOS processis employed with a fluorine doped nitride surface treatment of thepolysilicon buffer layer. Although a PBLOCOS process utilizing apolysilicon buffer layer is illustrated, it will be understood withbenefit of the present disclosure that the benefit of a fluorine-dopednitride surface treatment may be realized when used in other processesand to when used to treat other substrate surfaces, for example, totreat a surface of a silicon dioxide layer in a LOCOS-based process.

In FIG. 2, a silicon dioxide pad layer 22 has been formed on substrate20. Pad layer 22 may be formed to a desired thickness using any suitablemethod known in the art. For example, pad layer 22 may be thermallygrown, or deposited by CVD. In one embodiment, pad layer 22 may have athickness of from about 50 Å to about 250 Å, alternatively about 160 Å,although thicknesses greater than about 250 Å and less than about 50 Åare also possible. As used herein, “substrate” means any semiconductorsubstrate including, but not limited to, a semiconductor wafer substratesuch as silicon or GaAs. It will be understood that a “substrate” mayinclude, among other things, a semiconductor wafer or a semiconductorwafer having various process layers formed on the wafer. As used herein,“layer” may be used interchangeably with “film”.

Referring now to FIG. 3, a polysilicon buffer layer 24 is showndeposited on pad oxide layer 22. Polysilicon buffer layer 24 may beformed using any suitable method known in the art using a CVD process.In one embodiment, buffer layer 24 may have a thickness of from about200 Å to about 1000 Å, alternatively about 700 Å, although thicknessesgreater than about 200 Å and less than about 1000 Å are also possible.

In FIG. 4, polysilicon layer 24 has been exposed to a fluorine-dopednitride surface treatment, for example a low pressure fluorine-dopedammonia plasma CVD process, to form fluorine-doped nitridizedpolysilicon interface 25. As shown in FIG. 4, fluorine-doped nitridefilm 26 has also been formed as a result of the surface treatment. Inthis embodiment, fluorine-doped film 26 may be formed to a thicknessfrom about 1 Å to about 40 Å, alternatively from about 10 Å to about 30Å, alternatively from about 20 Å to about 25 Å, and furtheralternatively about 23 Å, using a treatment gas comprising a mixture ofcarbon hexafluorine and ammonia. In this regard, flow rate of carbonhexafluorine may be controlled to achieve a treatment gas having arelative volume or ratio of carbon hexafluorine to ammonia as describedelsewhere herein. For example, in a Novellus “CONCEPT II” reactor, flowrate of carbon hexafluorine may range from about 1 standard cubiccentimeters per minute (sccm) to about 300 sccm, alternatively fromabout 20 sccm to about 200 sccm, alternatively from about 20 sccm toabout 100 sccm, alternatively from about 20 to about 40 sccm, andfurther alternatively about 30 sccm; and simultaneous flow rate ofammonia may range from about 100 sccm to about 3000 sccm, alternativelyfrom about 100 sccm to about 500 sccm, alternatively from about 200 sccmto about 400 sccm, and further alternatively about 300 sccm.

Following fluorine-doped surface treatment of polysilicon layer 24, aconventional nitride surface treatment may be performed, either in aseparate reactor and/or processing step, or in the same reactor byterminating flow of carbon hexafluorine. As shown in FIG. 5,conventional nitride surface treatment with undoped ammonia plasmaresults in formation of undoped silicon nitride film 28 on top offluorine-doped nitride film 26. Thickness of film 28 may vary relativeto film 26 as described elsewhere herein. In one exemplary embodiment,undoped nitride film 28 may be formed to a thickness of from about 700 Åto about 3000 Å, alternatively from about 1200 Å to about 1800 Å, andfurther alternatively about 1550 Å.

Following nitride surface treatments, active region patterns 42 may bedefined on upper surface of silicon nitride layer 30, typically usingphotoresist. In this regard, any photolithographic or other suitablemethod for forming a pattern of active device areas may be employed. Asshown in FIG. 6, after photoresist is applied silicon nitride filmlayers 26 and 28 may be removed along with polysilicon buffer layer 24,for example, by anisotropic etching to form field oxide area 40 betweenactive areas 42.

Next, as shown in FIG. 7 field oxide region 44 may be formed onsubstrate 20 within field oxide area 40. First “bird's beak” areas 46are present at the surface of substrate 20, but significantly, no second“bird's beak” exists. Field oxide region 44 may be formed using anysuitable method known in the art for forming field oxides, including awet oxidation process, etc. In one embodiment, thickness of field oxide44 may be from about 1000 Å to about 10,000 Å, alternatively from about5000 Å to about 9000 Å.

As shown in FIG. 8, following formation of field oxide regions 44, filmlayers 28 and 26 may be removed in active areas 42 to expose polysiliconbuffer layer 24, for example, by chemical etching, HF acid stripping,reactive ion etching, etc. After removal of layers 28 and 26,polysilicon buffer layer 24 and oxide pad layer 22 may be removed inactive areas 42, for example, using a dry etching process in one or moresteps so as to expose substrate 20 of active region 42, as shown in FIG.9.

Using embodiments of the disclosed method, nonuniformity of nitridizedsubstrate interfaces may be decreased, resulting in increased processcapability. In one exemplary embodiment of the disclosed method, filmwithin wafer thickness nonuniformity (i.e., standard deviation/averagewafer thickness) of a nitride treated polysilicon test wafer may bereduced to values as low as about 2% or less, from values of about 7% ormore typically seen for surface treatments performed with conventionalundoped ammonia plasma treatments.

Table 1 illustrates the difference in film uniformity (i.e., thicknessvariation) between a standard undoped silicon nitride film formed byconventional ammonia plasma, and a fluorine-doped silicon nitride filmformed by a fluorine-doped ammonia plasma, both measured on a 8″ baresilicon test wafer, for one embodiment of the disclosed method.

TABLE 1 Standard NH₃ Film vs. C₂F₆- Doped Film Standard NH₃ Plasma C₂F₆-Doped Plasma C₂F₆ Flow (sccm) 0 30 Thickness (Å) 24 22 ThicknessVariation 10 <2 Range (Å) Wet Oxide Growth (Å) 65 65

Although one exemplary embodiment of the use of a fluorine-doped nitridesurface treatment in the creation of a MOS device using a PBLOCOSprocess has been described above, it will be understood with benefit ofthe present disclosure that embodiments of the disclosed method may bebeneficially employed in any method and/or for the creation of anydevice in which increased nitride uniformity and/or suppression of“second bird's beak” formation is desired.

EXAMPLES

The following examples are illustrative and should not be construed aslimiting the scope of the invention or claims thereof.

Example 1 C₂F₆ Doped Nitride Surface Treatment

In Example 1, a bare silicon wafer substrate was surface treated usingan ammonia PECVD process. FIG. 10 illustrates the reactor set-uputilized, including the spindle puming area 60, wafer position 62, andpumping area limit 64. The process was conducted in a “NOVELLUS CONCEPTII” reactor. The reactor was modified by replacing the mass flowcontrollers with smaller units acceptable for controlling carbonhexafluorine gas flows of as low as at least 10 sccm. Ammonia andfluorine gases were introduced through separate flow controllers.

In the various tests of this example, CVD treatment gas flow ratesranged from 100 sccm to 200 sccm of C₂F₆, and from 300 sccm to 400 sccmNH₃. The plasma recipe used in this example is given in Table 2.

TABLE 2 C₂F₆ Doped NH₃ Plasma Recipe (NOVELLUS CONCEPT II Reactor)Parameter Value N2 manifold A (sccm) 3000 N2 manifold B (sccm) 3000 NH₃(sccm) 300 C₂F₆ (sccm) 30 HFRF (watts) 250 LFRF (watts) 350 loPr (torr)1.9 Pres (torr) 2 hiPr (torr) 2 Temp (° C.) 400 SDT (sec) 12 ± 5 PCT(sec) 120 PreA (sec) 1 PosA (sec) 0.5 Soak (sec) 10 LLwt (sec) 3 flOK(%) 5 Tem % 2 TemT (sec) 1200 depR (A/min) 500 echR (A/min) 2000 PCER(A/min) 2000 pRes (torr) 2.3 SIHE (boolean) 0 WAIT (sec) 5 smFF(boolean) 1 pcLm (A) 20000 SPCT (sec) 60 NH₃E (boolean) 1 dfFF (boolean)0

Thickness and thickness variation was evaluated using a TencorInstruments “PROMETRIX UV-1250SE.” Results are given in Table 3.Abbreviations used in Table 2 and elsewhere herein are defined asfollows:

TABLE 3 Initial Test of C₂F₆ Doped NH₃ Plasma C₂F₆ NH₃ Wafer OxideParticles Flow Flow *SDT Thickness Thickness Growth, Post Plasma Slot(sccm) (sccm) (sec) (Å) Range (Å) (Å/minute) (number)  5 100 300 10.424.66 1.36 340 1  6 100 300 10.4 24.49 1.43 340 2  7 100 300 10.4 24.162.18  8 100 300 10.4 24.42 1.49  9 100 300 10.4 24.54 1.49 10 100 30020.4 26.29 1.58 11 100 300 20.4 26.28 1.56 12 200 300 10.4 19.66 3.61 13200 300 10.4 19.31 2.69 14 100 400 10.4 24.26 1.41 15 100 400 10.4 24.831.65 “SDT” = standard deposition time (seconds)

The results of Example 1 indicate that relatively low flow rates of C₂F₆result in more desirable oxide growth rates than higher flow rates.

Example 2 C₂F₆ Doped Nitride Surface Treatment with Lower Flow Rates ofC₂F₆

In Example 2, lower flow rates of C₂F₆ in the CVD treatment gas wereutilized to examine the effect on nitride thickness range and oxidegrowth. In this example, carbon hexafluorine flow rates ranged from 0 to100 sccm, while ammonia flow rate was maintained constant at 300 sccm.The experimental apparatus and test procedure of Example 1 were employedfor Example 2.

TABLE 4 Lower C₂F₆ Flow Rates C₂F₆ NH₃ Thick- Oxide Flow Flow SDT nessThickness Growth, Slot (sccm) (sccm) (sec) (Å) Range (Å) (Å/minute) 1100 300 10.4 21.25 0.63 925 2 100 300 10.4 20.84 0.95 923 3 0 300 10.425.41 9.23 110 4 0 300 10.4 25.51 9.39 113 5 60 300 10.4 22.38 0.49 8506 60 300 10.4 22.13 0.47 834

Many defects were observed (using the “Tencor 6220”) on the C₂F₆-dopednitride treated wafers after SiN treatment The defects were actuallysurface roughness. The particle map matched the 49 pt thicknessuniformity map seen after ammonia plasma deposition.

The results of Example 2 indicate that relatively low flows of C₂F₆produced desired uniformity benefit while not adversely affecting oxidegrowth.

Example 3 C₂F₆ Doped Nitride Surface Treatment with Lower Flow Rates andwith D-FEG Setup

In Example 3, the procedure of Example 2 was repeated, this time using adual frequency e-ground (“D-FEG”) reactor setup. Results of Example 3are shown in Table 5. In this example, carbon hexafluoride flow ratesranged from 0 sccm to 30 sccm while ammonia flow rate was maintained at300 sccm.

TABLE 5 Lower C₂F₆ Flow Rates with D-FEG Setup Oxide C₂F₆ NH₃ Thick-Growth, Flow Flow SDT ness Thickness (Å/minute) Slot (sccm) (sccm) (sec)(Å) Range (Å) (Avg./Max.) 1  0 300 8.5 25.49 6.65 88/340 2  0 300 8.525.64 7.35 83/291 3 30 300 8.5 21.25 1.39 68/117 4 30 300 8.5 20.84 1.4865/104

The results of this example indicate that maximum oxide growth rate waslower when C₂F₆ was introduced at the rates indicated.

FIGS. 11 and 12 are contour maps of silicon nitride film thickness forthe wafers of slots 1 and 3, respectively. As may be seen by the contourmaps, the wafer of slot 3 (surface treated with fluorine-doped ammoniaplasma) exhibited a within-wafer thickness variation range of only 1.39Å over a wafer mean film thickness of 24.19 Å, and had a standarddeviation of 0.3329 Å, giving a wafer thickness nonuniformity of 1.376%.This is much more uniform than the 6.65 Å within-wafer thickness rangeexhibited by the wafer of slot 1 (surface treated with conventionalundoped ammonia plasma), which had a mean thickness of 25.49 Å, astandard deviation of 1.9669 Å, and a wafer thickness nonuniformity of7.717%.

The film uniformity results of Examples 2 and 3 show that a much moreuniform interface between the substrate and nitride is achieved when anitridized interface is created using a fluorine doped ammonia plasmasurface treatment. These results also suggest that there is a residencetime factor that is not as important when carbon hexafluorine is presentin the treatment gas. While not wishing to be bound by theory, such afactor could be a mechanism related to mass transport of reactant ionsto the surface, or removal of byproducts. In addition it is possiblethat highly electronegative fluoride ions modify the ammonia plasma byscavenging hydrogen to form HF, and thus allowing a higher density forreaction with the silicon.

While the invention may be adaptable to various modifications andalternative forms, specific embodiments have been shown by way ofexample and described herein. However, it should be understood that theinvention is not intended to be limited to the particular formsdisclosed. Rather, the invention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims. Moreover, the differentaspects of the disclosed compositions and methods may be utilized invarious combinations and/or independently. Thus the invention is notlimited to only those combinations shown herein, but rather may includeother combinations.

What is claimed is:
 1. A method of depositing a substantially uniformfluorine-doped nitridized layer on an upper surface of a substrate in achemical vapor deposition process, comprising: exposing a surface of thesubstrate to a gas mixture comprising a fluorine-containing gas providedat a first steady flow rate and a nitrogen-containing gas provided at asecond steady flow rate for a period of time sufficient enough to form afluorine-doped nitridized layer having an interface with the substratesurface.
 2. The method of claim 1, wherein said substrate surfacecomprises silicon dioxide.
 3. The method of claim 1, wherein saidsubstrate surface comprises polysilicon.
 4. The method of claim 1,wherein the nitrogen-containing gas is at least one of ammonia,nitrogen, or a mixture thereof; wherein the fluorine-containing gas isat least one of C₂F₆, C₃F₈, CF₄, or a mixture thereof; and wherein saidexposing the substrate surface to the gas mixture occurs in a lowpressure plasma enhanced chemical vapor deposition process.
 5. Themethod of claim 1, wherein the nitrogen-containing gas is ammonia;wherein the fluorine-containing gas is carbon hexafluorine; and whereinsaid exposing the substrate surface to the gas mixture occurs in a lowpressure plasma enhanced chemical vapor deposition process.
 6. Themethod of claim 1, further comprising exposing the fluorine-dopednitridized layer to an undoped treatment gas comprising a nitrogencomponent for a period of time sufficient enough to form an undopednitridized layer on the fluorine-doped nitridized layer.
 7. The methodof claim 6, wherein the nitrogen-containing gas of the doped gas mixtureis ammonia; wherein the fluorine-containing gas of the doped gas mixtureis carbon hexafluorine; wherein the nitrogen component of the undopedtreatment gas is ammonia; and wherein said exposing the substratesurface to the doped gas mixture and said exposing the fluorine-dopednitridized layer to the undoped treatment gas each occurs in arespective low pressure plasma enhanced chemical vapor depositionprocess.
 8. The method of claim 5, wherein the fluorine-doped nitridizedlayer comprises a thickness within the range of about 1 Å to about 40 Å.9. The method of claim 5, wherein the fluorine-doped nitridized layercomprises a thickness within the range of about 10 Å to about 30 Å. 10.The method of claim 5, wherein the doped gas mixture comprises a gasflow rate ratio of carbonhexafluorine to ammonia that is within therange of about 1:1 to about 1:20.
 11. The method of claim 6, wherein thefluorine-doped nitridized layer comprises a thickness of from about 1 Åto about 40 Å; and wherein the undoped nitridized layer comprises athickness within the range of about 700 Å to about 3000 Å.
 12. Themethod of claim 7, wherein the substrate comprises a semiconductorwafer, and wherein the undoped nitridized layer has a within waferthickness variation of less than about 2 Å.